Large grain cobalt-samarium intermetallic permanent magnet material stabilized with zinc and process

ABSTRACT

AN ALLOY OF COBALT AND SAMARIUM IS CAST TO PRODUCE A CAST BODY HAVING GRAINS RANGING IN SIZE FROM ABOUT 100 MICRONS TO 1000 MICRONS. THE CAST BODY IS ANNEALED IN AN ATMOSPHERE IN WHICH IT IS SUBSTANTIALLY INERT AT A TEMPERATURE RANGING FROM ABOUT 900*C. UP TO A TEMPERATURE BELOW ITS MELTING POINT FOR A PERIOD OF TIME RANGING FROM ABOUT 5 MINUTES TO 24 HOURS. TO GET SIGNIFICANTLY USEFUL PERMANENT MAGNET PROPERTIES, IT SHOULD BE ANNEALED AT A PARTICULAR ANNEALING TEMPERATURE FOR A PERIOD OF TIME SUFFICIENT FOR THE RESULTING FREE GRAINS TO SHOW AT ROOM TEMPERATURE, AFTER BEING MAGNETIZED TO AT LEAST APPROACH SATURATION MAGNETIZATION, A RELATIVE MAGNETIZATION VALUE OF AND/OR OF AT LEAST 50 PERCENT AT A DEMAGNETIzING FIELD OF -4 KILOOERSTEDS WITH RELATIVE MAGNETIZATION 4NJBR, BY DEFINITION, BEING 1,00 AT ZERO DEMAGNETIZING FIELD. THE ANEALED BODY IS THEN COMMINUTED TO A GRAIN SIZE CORRESPONDING TO OR SMALLER THAN THE GRAIN SIZE OF THE CAST BODY AND RANGING FROM ABOUT 50 MICRONS TO 200 MICRONS. THE FREE ANNEALED GRAINS ARE AMIXED WITH 1-15% BY WEIGHT ZINC POWDER AND THE RESULTING MIXTURE IS HEATED TO MELT THE ZINC POWDER TO FORM A CONTINOUS COATING OF ZINC ON THE INDIVIDUAL GRAINS.

May 21, 1974 M.G. BENZ 3,811,962

LARGE GRAIN COBALT-SAMARIUM INTERMETALLIC PERMANENT MAGNET MATERIAL STABILIZED WITH ZINC AND PROCESS Filed April 17, 1972 PACKING= L00 8 MAGNET/2A now 47.1 mos/lass l6 l4 l2 --l0 8 6 4 I 2 O DEMAG/VET/Z/NG FIELD, kOe

nited States Patent "O US. Cl. 148-103 13 Claims ABSTRACT OF THE DISCLOSURE An alloy of cobalt and samarium is cast to produce a cast body having grains ranging in size from about 100 microns to 1000 microns. The cast body is annealed in an atmosphere in which it is substantially inert at a temperature ranging from about 900 C. up to a temperature below its melting point for a period of time ranging from about 5 minutes to 24 hours. To get significantly useful permanent magnet properties, it should be annealed at a particular annealing temperature for a period of time sufficient for the resulting free grains to show at room temperature, after being magnetized to at least approach saturation magnetization, a relative magnetization value of 41r1/ B of at least 50 percent at a demagnetizing field of -4 kilooersteds with relative magnetization 41rI/B,, by definition, being 1.00 at zero demagnetizing field. The annealed body is then comminuted to a grain size corresponding to or smaller than the grain size of the cast body and ranging from about 50 microns to 200 microns. The free annealed grains are admixed with 1-15% by weight zinc powder and the resulting mixture is heated to melt the zinc powder to form a continuous coating of zinc on the individual grains.

The present invention relates generally to the art of permanent magnets. In one aspect, it is concerned with making large grain cobalt-samarium intermetallic permanent magnet material having unique permanent magnet properties which are stabilized with zinc. In another aspect it is concerned with permanent magnets comprised of the novel zinc-coated grains bonded to a non-magnetic making permanent magnets has been developed based on cobalt and rare-earth elements, particularly cobalt and samarium. The improvement over prior art materials is so great that the cobalt-rare earth magnets stand in a class by themselves. In terms of their resistance to demagnetization the new materials are superior to convenvtional magnets of the Alnico and ferrite type, and their magnetic energy is significantly greater. Since more powerful the magnet for a given size is the smaller it can be for a given job, the cobalt-rare earth intermetallic magnets have applications for which prior art materials not even be considered.

Permanent magnet properties of bulk cobalt-rare earth intermetallic bodies are enhanced by reducing them to a powder. The as-ground powder can be incorporated in canbonding media to produce a composite finished permaproperties of the material fall significantly. Specifically, direct reduction of a cobalt-samarium intermetallic bulk body to grains having a size as low as 50 microns, results in a material having such poor properties as to b useless for permanent magnet applications.

There are a number of disadvantages inherent in the use of a cobalt-samarium intermetallic powder having a particle size as low as 10 microns or lower. When this powder is exposed to air, particularly at temperatures slightly above room temperature, its intrinsic coercive force decreases irreversibly at a significant rate. This decay in coercive force substantially diminishes the advantages to be gained by converting the bulk cobaltsamarium intermetallic body to a powder. Also, preparation of powders of such fine size presents a number. of handling problems and is time-consuming and costly.

In copending US. Patent Application Ser. No. 244,424 entitled, Large Grain Cobalt-Samarium Intermetallic Permanent Magnet Material And Process, filed of even date herewith in the name of Mark G. Benz and assigned to the assignee hereof, and which by reference is made part of the disclosure of the present invention, there is disclosed an invention which overcomes the aforementioned disadvantages by providing a novel large grain cobalt-samarium intermetallic permanent magnet material having permanent magnet properties useful for a wide range of permanent magnet applications. These permanent magnet properties are not shown by the asground material of the same size. The time-consuming handling operations of transforming the material to a powder are eliminated. In comparison to the prior art powder, these grains are easier to handle and they are more stable since they oxidize much more slowly. In addition, they show permanent magnet properties which are as good or better than the prior art powder. Because these grains are significantly larger than the powder of the prior art, they are also easier to orient magnetically since magnetic alignment depends on a torque situation.

The permanent magnet properties of the disclosed grains of the aforementioned copending patent application, however, tend to deteriorate in air at high temperatures, i.e. temperatures of about 150 C. The present application provides a method for stabilizing the permanent magnet properties of these grains in air at temperatures ofabout 150 C.

Those skilled in the art will gain a further and better understanding of the present invention from the detailed description set forth below, considered in conjunction ing grains ranging in size from about microns to 1000 microns. The cast body is annealed in an atmosphere in which it is substantially inert at a temperature ranging from about 900 C. up to a temperature below its melting point for a period of time ranging from about 5 minutes to 24 hours. Generally, the annealing temperature ranges from about 900 C. to 1200 C. since no significant improvement in magnetic properties is produced at temperatures significantly higher than 1200 C. Annealing time for a particular annealing temperature depends on the particular permanent magnet properties desired. Specifically, to get significantly useful permanent magnet properties, it should be annealed at a particular annealing temperature for a period of time sufficient for the resulting free grains to show'at room temperature, after being magnetized to at least approach saturation magnetization, i.e. within about 10 percent of full saturation magnetization, a relative magnetizationwalue of 41rl/B, of at least 50 percent at a demagnetizing field of---4 kilooersteds-with relative magnetization 41rJ/B by definition, being 1.00 at zero demagnetizing field. The annealed body is then comminuted to a grain size corresponding to or smaller than the grain size of the cast body and ranging from about 50 to 200 microns. Alternatively, prior to annealing, the cast body can be comminuted to a'grain size corresponding to its grain size or smaller than its grain size and the resulting free grains, ranging in size from about 50 to 200 microns, are annealed at a temperature ranging from about 900 C. to 1200 C. for a period of time ranging from about 5- minutes to 24 hours. Again, annealing time for a particular annealing temperature depends on the particular permanent magnet properties desired, and for significantly useful permanent magnet properties, the free grains should -be annealed for a period of time sufiicient for the annealed free grains to show at room temperature, after being magnetized to at least approach saturation magnetization, i.e. within about percent of full saturation magnetization, a relative magnetization value of 411'] /B of at least 50 percent at a demagnetizing field of 4 kilooersteds with relative magnetization 41rJ/B,, by definition, being 1.00 at Generally, the best magnetic properties are produced at annealing temperatures ranging-from 1100 -C. to

.zero demagnetizing field. The free annealed grains are admixed with zinc powder in an amount of about 1% to 15% by weight of said grains and the resulting mixture is heated to melt the zinc powder to form a continuous coating of zinc on the individual grains.

The cobalt-samarium alloy of the present invention contains samarium in an amount of about 34 to 38 percent by weight of the alloy, and generally, to attain the best magnetic properties, it contains samarium in an amount of about 35 percent by weight of the alloy. Grains produced in accordance with the present process but having a cobaltsamarium composition outside this range do not produce satisfactory permanent magnets. The alloy is prepared in an atmosphere in which cobalt and samarium are substantially inert such as a noble gas or under a vacuum by a number of methods such as, for example, by induction or arc melting the cobalt and samarium. The molten alloy should, preferably, also be cooled in an atmosphere in which it is substantially inert such as a noble gas or under a vacuum.

The cobalt-Samarium alloy is cooled at a rate sufiiciently slow to produce a solid cast body wherein the grains range -in size from 100 to 1000 microns. This can be determined empirically using standard metallurgical techniques such as, for example, casting a liquid melt in a heated mold or simply allowing the molten alloy to cool in a crucible at room temperature. To prevent oxidation, cooling should be carried out in an atmosphere in which the alloy is substantially inert such as a noble gas or a vacuum. The solid cast body should have grains with a minimum size of about 100 microns since it would be difiicult and not practical to 'try to obtain the required amount of the present single are smaller than 100 microns.

The large grain solid cobalt-Samarium intermetallic cast body can then be annealed or, alternatively, the cast body can be comminuted and the resulting free grains annealed. Annealing is carried out in an atmosphere in which the cobalt-samarium intermetallic material is substantially inert such as argon or in a vacuum. If the body is comminuted to free grains prior to annealing, the free grains also should be annealed in a container made of a material to which it is substantially inert such 'as molybdenum, tantalum or niobium to prevent contamination. Annealing can be carried out at a temperature ranging from about 900 C. up to a temperature below the melting point 1200 C. The particular annealing time period for a particular annealing temperature depends largely on the specific permanent magnet properties desired. To produce significantly useful permanent magnet properties, it should be sufficiently long to produce an annealed material having the inherent property of showing at room temperature, after being magnetized to at least approach saturation magnetization, a relative magnetization value 41rl/B, of at least 50 percent at a demagnetizing field of--4 kilooersteds. Generally, the longer the material is annealed, the higher its relative magnetization value becomes at higher demagnetizing fields, i.e. demagnetizing fields of --4 kilooersteds and higher. For example, in the present process, annealing the cobalt-samarium alloy at a temperature ranging from about 1100 C. to 1200 C. for a period of 10 hours should produce free grains having a relative magnetization value of at least 50 percent or 0.5 at a demagnetizing field of 10 kilooersteds. Generally, after an annealing period of 24 hours. no significant improvement in permanent magnet properties occurs.

The term relative magnetization as used herein is the ratio of magnetization 41rJ to remanent induction B,.. Specifically, when a magnetic field is applied to a permanent magnet material, a magnetization value of 41r] gauss is established therein. When the magnetic field is removed, the material has a remanent induction 8,. The intrinsic coercive force H is the field strength at which the mag netization 41rJ is zero and is a measure of a permanent magnets resistance to demagnetization. An additional measure of a permanent magnets resistance to demagnetization, and one which is useful in defining the permanent magnet properties of the present free grains, is the shape of the hysteresis loop in the second quadrant wherein magnetization 41] or relative magnetization 41rJ/B verses a negative field H which shows what positive values of magnetization can be maintained in the presence of the demagnetizing field H. Specifically, the more square this second quadrant curve is, the higher is the magnetization or relative magnetization at a particular negative or demagnetizing field H, and the greater is the resistance of the magnet to demagnetization at such demagnetizing field It has been determined that the present cobalt-samarium alloy in solid bulk form has a saturation magnetization 41rJ value of about 9000 to 11,000 gauss. This is the maxiumum magnetization value achievable for this cobalt-samarium composition in solid bulk form. Theoretically, in the ideal situation, free grains of this cobaltsamarium alloy, when incorporated in a non-magnetic matrix to a volume fraction of about one-half, having an alignment factor of 1.00 and magnetized to saturation, should have a saturation magnetization 41rJ of about 4500 gauss to 5500 gaus, a remanent induction B of about 4500 gauss to 5500 gauss and maintain a magnetization value of about 4500 gauss to 5500 gauss at a demagnetizing field of about 4 kilooersteds. In the present invention, when the free grains are incorporated in the non-magnetic matrix to a volume fraction of about one-half and magnetically aligned therein along their easy axis of magnetization so as to have an alignment factor of about 0.95 and magnetized to saturation or approaching saturation magnetization, i.e. within about ten percent of full saturation magnetization the resulting permanent magnet has typically, a magnetization value 41rJ of about 4000 gauss at a demagnetizing field of -4 kilooersteds. On the other hand, for significantly useful permanent magnet properties, the present free grains incorporated in a non-magnetic matrix to a volume fraction of one-half, i.e. comprising one-half by volume of the permanent magnet, and magnetized to sat The rate at which the annealed material is cooled is not critical and a number of conventional techniques can be used which do not oxidizethe material to any significant extent. Preferably, the annealed material is cooled in an atmosphere in which itis substantially inert such as, for example, argon or nitrogen, or it may be cooled in a vacuum, and generally, it is cooled to room temperature.

The cast body can be comminuted to free grains by a number of conventional methods, such as, for example, by crushing by mortar and pestle, double disc pulverizer, or jaw crushers. Comminution is preferably carried out in an atmosphere in which the material is substantially inert such as argon'or under a vacuum.

In the present invention, the grains of the cast cobaltsamarium body are single crystals and the cast body is comminuted to free grains having a size corresponding to the grain size of the cast body, or it is comminuted to free grains having a size smaller than the grain size of the cast body. Specifically, the free grains have a size ranging from about 50 microns to 200 microns. Free grains having a size significantly larger than 200 microns do not have useful permanent magnet properties. In addition, comminution should be carried out so that a major portion, i.e. at

least 85 percent by weight of the free grains, are single crystal free grains. The single crystal structure of the free grains is determinable by standard metallographic tech niques such as, for example, X-ray diffraction techniques. Preferably, at least 95 percent by weight or substantially all of the resulting free grains are single crystal grains. Since the Weakest bonds in the cast body exist at the grain boundaries, it is at these boundaries that breakage of the cast body usually preferentially occurs during comminution. In practice, due to breakage, the cast body should preferably have a grain size larger than that desired for the free grains to produce the highest amount of free grains of a single crystal.

The free annealed grains are admixed with zinc pow der, preferably at room temperature, in an amount of about 1% to 15% by weight of said free annealed grains which range in size from about 50 microns to 200 microns. For most applications, the zinc is used in an amount of about 1-5% by weight of said grains. Since the smaller sized free grains have a larger total surface area, they generally require the larger amounts of zinc powder. Preferably, to produce a substantially thorough mixture, the zinc powder should not be larger than about 150 microns, and generally, ranges in size from about 40 to 100 microns. The annealed free grains and the zinc powder can be mixed in 'air'by a number of conventional techniques such as simply stirring together or tumbling. Occasion-. ally, ,if desired, a liquid in which the mixture is substantially inert such as isopropyl alcohol can be included in the mixture to promote the production of a thorough mixture. The resulting mixture is heated in an atmosphere in which it is substantially inert at a temperature which fuses or melts the zinc powder which, in the present invention, generally ranges from about 400 C. to about 475 C., and preferably, it is about 450 C. Temperatures higher than 500 C. should not be used since they would tend to diffuse the zinc into the grains too rapidly. The mixture is heated for a period of time sufiicient to form a continuous coating of zinc on the individual grains but insufficient to diffuse the zinc into the grains to deterio rate their permanent magnet properties to any significant extent. This time period can be determined empirically, and generally, at temperatures ranging from about 425 C. to about 450 C., the time period may range from about 15 to about 60 minutes.

The zinc coating formed on the grains should be sufliciently thick to maintain the magnetic stability of the grains in air at elevated temperatures of about 150 C.

This means that the thickness of the zinc coating should be suflicient to prevent air from penetrating to the cobalt-samarium grain surface. Specifically, when the'zinccoated grains are in air, the outer portion of the zinc coating should be available to be oxidized by the air leaving an inner continuous zinc coating to maintain the stability of the permanent magnet properties of the grains in air at elevated temperatures. Generally, in the present invention, grains having a size of microns coated with zinc in an amount of 5% by weight of the grains have a zinc coating 2.5 microns in thickness. The use of the zinc in an amount of 1% to 15% by weight of the free grains has substantially no deteriorating effect on the permanent magnet properties of the grains and in some instances improves these properties. Amounts of zinc significantly in excess of 15 by weight of the free grains does not improve magnetic stability and would prevent a close packing of the grains in the non-magnetic matrix thereby diluting the permanent magnet properties somewhat.

The zinc-coated free grains of the present invention are incorporated in a non-magnetic matrix to form permanent magnets. To produce satisfactory magnetic alignment of the grains, the zinc-coated grains are incorporated into the non-magnetic matrix and, while the matrix is kept in a condition sufficiently liquid to maintain the grains in a substantially unlocked position, an aligning magnetizing field is applied to the incorporated grains to align them substantially along their preferred axis of magnetization which is the C or easy axis of magnetization, and, if desired, also to magnetize them as required. Specifically, since the grains are substantially unlocked in position, the incorporated single crystal grains subjected to the magnetizing field will be able to turn in a direction most favorable from a magnetic point of view, i.e. align along their easy axis of magnetization. While the magnetically aligned zinc-coated grains are still subject to the aligning magnetizing field, which should be at least 4 kilooersteds to produce satisfactory alignment, i.e. an alignment factor of at least about 0.85, the non-magnetic matrix is solidified to bond the grains and lock them in their magnetically aligned position. As used herein, the alignment factor is the ratio of the remanent induction B to the saturation magnetization 41rI multiplied by the volume packing fraction p. That is, A=B,/41rJ -p. Frequently, in practice, an additional magnetizing field may be applied to the locked aligned grains to magnetize them to full saturation or to approach saturation magnetization and the specific strength of this magnetizing field depends largely on the degree of alignment of the grains. Generally, where the present grains have an alignment factor of at least about 0.85, such a magnetizing field ranges from about 10 kilooersteds to 100 kilooersteds. In another techniqueyif desired; the zinc-coated free grains of the present invention can be magnetized to approach saturation, then incorporated in the liquid nonmagnetic matrix,,and an aligning magnetic field applied to the incorporated magnetized grains to align them along their easy axis of magnetization before the matrix is solidi fraction of about 50 percent by volume.

Permanent magnets having useful permanent magnet properties for a wide range of applications are produced when the zinc-coated grains of the present invention are incorporated in a non-magnetic matrix and magnetized. Specifically, the resulting permanent magnets have a useful substantially stable magnetization 411-1 in air at temperatures as high as about C. The permanent magnets of the present invention 'areuseful in telephones, electric clocks, radios, television, and phonographs. They are also useful in portable appliances, such as electric toothbrushes and electric knives, and to operate automobile ac- .cessories. In industrial equipment, the present permanent magnets can be used in such diverse applications as meters 1 Relative magnetization 471' J/B,:

and instruments, magnetic separators, computers and migrains were transferred to a chamber having an atmoscrowave devices. phere of argon at room temperaturewhere they were All parts and percentages used herein are by weight uncooled to room temperature. As indicated in the followless otherwise noted. ing table, a portion of the annealed product was used for The inventlon is further illustrated by the following ex- Samples 1, 2, 3 and 14. v amples in which, unless otherwise noted, the conditions The remaining portion of annealed grains were treated and procedure were as follows: with zinc as indicated to form Samples4-12 and l 8.

The grain structure of the solid cast cobalt-samarlum Specifically, the zinc powder used for all, these samples body was dete mined by slic ng oil a portlon of the caSthad a size of about 50 microns. The indicated amount of 11g, pol shing 1t and examining it under a microscopezinc powder was stirred with the annealed grains in air The size of the free grams 'was determined by standard 15 at room temperature and occasionally with the inclusion technlqlles 1 8 Us Standard Screen Y of a small amount of isopropyl alcohol to improve wetting All magnetic measurements were carried out at room of h in powder onto the grains to produce a' thorough temperature. 1 mixture. Each resulting sample mixture was heated in an Under the condit ons set forth 1n the following examatmosphere of ur argon at the temperature and for the p the resultmg ahgnmentifactor was at least about 0.85 time eriod indicated in the following table. Upon comd the gramswere a h g to at least pp h satw pletion of the heating, each sample was transferred to a I'alZlOIl magnetlzat on ne. Wlthlll about 10 percent of full chamber having an atmosphere of argon at room tem- Salllfatlon magnetllatlonperature Where it cooled to room temperature. Samples EXAMPLE 1 4-12 and 15-18 appeared to have a continuous coating of Zinc thereon.

About 500 grams of an alloy melt of 63% cobalt and To determine the magnetic properties of each sample of 37% samarium was prepared by induction-melting under grains, the grains were incorporated into a body of liquid argon in an alumina crucible which had an inner diamparafiin wax in a small glass tube to a fraction of about eter of about 2 inches and was about 3% inches high. 50% by volume. The wax was sufiiciently liquid so that The liquid melt filled about one-half of the crucible and the grains were substantially unlocked in position. In all was maintained in an argon atmosphere at room temperaof the samples tabulated in the following table, an align- .ture to slowly solidify. To recover the resulting solid cast ing magnetic field ranging from about 15.5 to about 18.5 alloy, the crucible was broken with a hammer. The grains kilooersteds was then applied to the incorporated sample in the cast alloy ranged in size from about 100 microns to align the grains along their easy axis of magnetization to 1000 microns. and the wax was cooled in the aligning magnetic field The solid cast alloy was comminuted in a nitrogen atuntil it solidified to lock the magnetically aligned grains mosphere by means of a double disc pulverizer and a in position. In Samples 13-18, there was applied to the batch of free grains having a size ranging from about 104 magnetically aligned locked grains, 9. magnetizing field microns to 147 microns was recovered therefrom with of 60 kilooersteds. about 95% of these recovered grains being single crystal Relative magnetization 41rJ/B, was measured at demagfree grains. A portion of this batch was used to form netizing fields starting from zero demagnetizing field. At Sample 13, the as-ground sample in the following table. zero demagnetizing field, relative magnetization. 41rJ/B The remaining portion of this batch of free grains was has, by definition, avalue of 1.00.

Magnetic Demagnetizing fields (koe) Sa field No Condition (koe) 1 -2 3 -4 -5 6 -7 -8 9 ---10 1.. Annealed 16 hr. 1,110 c. (Argon) 15.5 .91 .82 .75 .59 .58 .45 .25 .11 0 17 2 Apnefizled 16 hr. 1,110 C. (Argon) plus 1 hr. 150 C. 15.5 .98 .83 .54 .22 '-.07 3 An iie aa led 16 hr. 1,110 C. (Argon) plus 18 hr. 150 C. 15. 5 .90 .11 -.34 V p 4 Arii igi'l d g hr. 1,110" c. (Argon) plus 1% zn 20min. 15.5 1.00 .99 .89 .79 .72 .54 .50 .40 .25 .15

11 5 Annealed 16 lil ".1,110 o. (Argon) plus 1% Zn 20 min. 15.5 .95 .94 .87 .81 .70 .53 .52 .43 .25 .14 450 C. (Argon) plus 18 hr. 150 0. (air). 6 Annealed 16hr.1,110 c. (Argon) plus 1% Zn 20 min. 18.5 .955 .92 .82 .71 .47 .11 -.22

450 C. (Argon) plus 18 hr. 150 0. (air). 7 Annealed 16 hr. 1,1l0 C. (Argon) plus 1% Zn 20 min. 15. 5 88 .64 22 16 450 C. (Argon) plus 66 hr. 150 0. (air). 8 Aillgpldg hr. 1,110 C. (Argon) plus 5% Zn 15 min. 17.5 1.01 .91 .87 .81 .76 .71 .67 .61 .55 .49

1' 011 9 g..- Ailggagdi fir. 1,110 O. (Argon) plus 5% Zn30min. 17.5 .96 .92 .85 .83 .75 .65 .57 .58 .52' .46

1' 011 10 Ailslbeagddf l ll. 1,110 C. (Argon) plus 5% Zn 60 min. 17.5 1.02 .98 .90 .87 .75 .67 .65 .60 .57 .45

0 11 Anuealed16 l lr.1,110 o. (Argon) plus 5% Zn 15 min. 17.8 .92 .92 .87 .79 .71 .52 .59 .49 .45 .40

450 C. (Argon) plus 1 hr. 150 C. (air). 12 Annealed 16 hr. 1,110 C. (Argon) plus 5% Zn 15 min. .90 .89 81 .74 69 .62 52 43 450 C. (Argon) plus 16 hr. 150 0. (air). 1 13 As ground--- .55 1.02

-- Annealed 16 hr. 1,110 C. (Argon) .94 .91 .90 .84 .73 .67 .57 .37 .24

15 Ailslae agdai hr.)1,110 C. (Argon) plus 5% Zn 15 min. .93 .92 .91 .90 .89 .87 .83 .85 .78

16 Annealed 16 r. 1,110 O. (Argon) plus 5% Zn 15 min. 60 .99 i .98 .97 .93 .91 .87 180 .72

450 C. (Argon) plus 1 hr. C. (at

1 Annealed 15 lir.1,110 o. (Argon) plus 5% Zn 15min. 50 .99 .98 97 .99 .95 .89 .81 .70

450 C. (Argon) plus 16 hr. 150 0. (air). I

Annealed 16 hr. 1,1l0 C. (Argon) plus 5% Zn 15 min. 60 1.04 .96 .92 .87 .82 .78 .63 .61 53 450 C. (Argon) plus 150 hr. 150 0. (air).

As tabulated in the above table, a comparison of the magnetic properties of the samples wherein the grains were coated with zinc with those not treated with zinc illustrates the present invention. Specifically, a comparison of Sample 1, which was not treated with zinc, with Samples 4 and 8-1-0, which were zinc-coated in accordance with the present invention, shows that the zinccoating on the grain improves the resistance to demagnetization of the material at demagnetizing fields ranging from -1 to -10 kilooersteds. This is also illustrated by a comparison of Sample 14 with Sample 15.

After 18 hours in air at 150 C., Sample 3, which was not treated with zinc, shows that its resistance to demagnetization fell significantly at a demagnetizing field of 3 kilooersteds, whereas Sample 6, which was zinc-coated in accordance with the present "invention, exhibited good resistance to demagnetization at a demagnetizing'field of -4 kilooersteds. Also, Sample 12, which was zinc-coated in accordance with the present invention, after 16 hours at 150 C. in air, showed good resistance to demagnetization at demagnetizing fields as high as 9 kilooersteds.

After being heated in air at 150 C. for periods of time ranging from 1 hour to 150 hours, Samples 5, 6, ll, 12, 16, 17 and 18, all of which were zinc-coated in accordance with the present invention, showed a resistance to demagnetization higher than 50% at a demagnetizing field of 4 kilooersteds which makes them useful for a wide variety of permanent magnet applications.

EXAMPLE 2 A magnetized sample of zinc-coated annealed free grains was prepared as set forth for Sample of Example 1 except that the zinc-coated annealed grains were packed to a volume fraction of .39 or 39 percent. v

This sample had an alignment factor A of 0.94, a remanent induction B, of 0.03 kG., a coercive force H, of 2.9 kOe., a maximum energy product (BH) of 2.3 MGOe., and an intrinsic coercive force H of 18 kOe.

The resistance to demagnetization of this sample is shown in the accompanying figure and shows what high magnetization values can be maintained at room temperature at relatively high demagnetizing fields. The resistance to demagnetization of the solid alloy of the same composition wherein the packing fraction is 1.00 or 100 percent, and which was also magnetized with a field of 60 kilooersteds, is shown for comparison.

US. Pat. 3,615,914 which issued Oct. 26, 1971 to Joseph '1. Becker and Robert E. Cech and which is as signed to the assignee hereof, discloses a method of producing particulate cobalt-samarium which is substantially stable in air which, briefly stated, comprises contacting the cobalt-samarium particulate material with zinc in an amount sufficient to stabilize the material without significantly reducing the coercive force at a temperature above the melting point of the zinc. This patent does not disclose the present annealed large grain cobalt-samarium inintermetallic permanent magnet material stabilized with zinc.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for treating large grains of cobalt-samarium alloy having useful permanent magnet properties to stabilize these properties in air at elevated temperatures which comprises forming an alloy melt of cobalt and samarium wherein the samarium content ranges from about 34 to 38 percent by weight of said alloy, cooling said alloy melt at a rate sufficiently slow to produce a solid cast body wherein the cast grains have a size ranging from about 100 microns to 1000 microns, said cast grains being single crystal grains, annealing said solid cast body in an atmosphere in which is substantially inert at a temperature ranging from 900 C. to 1200" C. for a period of time ranging from about 5 minutes to 24 hours, and comminuting said annealed cast body to produce free grains having a size corresponding to or smaller than the size of said cast grains, recovering said free grains ranging in size from about 50' to 200 microns with at least percent by weight of said recovered free grains being single crystal. free grains, said annealing being carried out so that said resulting recovered free grains have the property of showing at room temperature after being magnetized to at least within about 10 percent of full saturation magnetization a relative magnetization value 4 1rJ/B, of at least 50 percent at a demagnetizing field of 4 kilooersteds, admixing said recovered free grains with zinc powders in an amount ranging from about 1 to 15 percent by weight of said grains, and heating the resulting mixture at a temperature ranging from 400 C. to 500 C. in an atmosphere in which said mixture is substantially inert to form a continuous zinc coating on said grains to produce zinc-coated free grains.

2 A process according to claim 1 wherein said annealing temperature ranges from about 1100 C. to 1200 C.

3. -A process according to claim 1 wherein at least percentwby weight of said recovered free grains are single crystal grains.

4. A process for preparing a solid permanent magnet which comprises incorporating said zinc-coated free grains of claim 1 in a non-magnetic matrix having a consistency which is sufficiently liquid to leave said zinc-coated grains in a substantially unfixed position, applying to said incorporated zinc-coated grains a magnetic field of at least 4 kilooersteds to align the grains along their easy axis of magnetization and fixing said magnetized grains in their magnetically aligned position by solidifying said non-magnetic matrix material. 5. A process according to claim 4 wherein said nonmagnetic matrix material is selected from the group consisting of plastics, elastomers, metals and wax.

6. A process for treating large grains of cobalt-samarium alloy having useful permanent magnet properties to stabilize these properties in air at elevated temperatures which comprises forming an alloy melt of cobalt and samarium wherein the samarium content ranges from about 34 to 38 percent by weight of said alloy, cooling said alloy melt at a rate sufliciently slow to produce a solid cast body wherein the cast grains have a size ranging from about microns to 1000 microns, said cast grains being single crystals grains, comminuting said solid cast body to produce free grains having a size corresponding to or smaller than the size of said cast grains, recovering said free grains ranging in size from about 50 to 200 microns with at least 85 percent by weight of said recovered free grains being single crystals free grains, annealing said free grains in an atmosphere in which they are substantially inert at a temperature ranging from 900 C. to 1200 C. for a period of time ranging from about 5 minutes to 24 hours, said annealing being carried out so that the annealed recovered free grains have the property of showing at room temperature after being magnetized to at least within about 10 percent of full saturation magnetization a relative magnetization 411-I/B of at least 50 percent at a demagnetizing field of -4 kilooersteds, admixing said recovered free grains with zinc powder in an amount ranging from about 1 to about 15 percent by weight of said grains, and heating the resulting mixture at a temperature ranging from about 400 C. to 500 C. in an atmosphere in which said mixture is substantially inert to form a continuous zinc coating on said grains to produce zinc-coated free grains.

7. A process according to claim 6 wherein at least 95 percent by weight of said recovered free grains are single crystal free grains.

8. A process for preparing a solid permanent magnet which comprises incorporating said zinc-coated free grains of claim 6 in a non-magnetic matrix having a consistency which is sufiiciently liquid to leave said zinc-coated grains in a substantially unfixed position, applying to said incorporated grains a magnetic field of at least 4 kilooersteds to align the grains along their easyaxis of magnetization and fixing said magnetized grains in their magnetically aligned position by solidifying said non-magnetic matrix material.

9. A process according to claim 8 whereinsaid nonmagnetic matrix material is selected from the group consisting of plastics, elastomers, metals, and wax.

10. Annealed free grains consisting essentially t cobalt-samarium alloy having substantially stable permanent magnet properties in air at elevated temperatures wherein the samarium content ranges from about 34 to 38 percent by weight of said cobalt-samarium alloy, said free grains ranging in size from about 50 microns to 200 microns with at least 85 percent by weight of said free grains being single crystal grains, said annealed free grains having a continuous coating of zinc with said zinc being present in an amount ranging from about 1 to percent by weight of said grains, said zinc-coated annealed free grains having the property of showing at room temperature after being magnetized to at least approach saturation magnetization a relative magnetization 41rJ/B of at least 50 percent at a demagnetizing field of -4 kilooersteds where 4x118 the magnetization value and B is the remanent induction.

11. Annealed free grains consisting essentially of cobaltsamarium alloy containing about percent by weight samarium, having substantially stable permanent magnet properties in air at elevated temperatures, said free grains ranging in size from about microns to microns with at least 95 percent by weight of said free grains being single crystal free grains, said annealed free grains having a continuous coating of zinc with said zinc being present in an amount ranging from about 1 to 15 percent by weight of said grains, said zinc-coated annealed free grains having the property of showing at room temperature after being magnetized to at least approach saturation magnetization a relative magnetization 41rJ/B of at least 50 References Cited UNITED STATES PATENTS 3,540,945

11/1970 Strnat et al 14831.57

.. 3,615,914 1-0/1971 Becker 148-101 13,421,889 1/1969 Ostertag et a1 148-105 3,516,612 6/1970 Fullman et al 148-105 3,684,591- 8/1972 Martin 148-101 12/1970 Buschow et a1 14831.57

' OTHER REFERENCES Westendorp, F. F., et al., Perm. Mags. With Energy Products of 20 Mil. G. Oer, in Sol. St. Comm., 1, 1969, pp. 639-40 (QClJ82).

Becker, J. J.,,A New Family ofCo-Bace Perm. Mag. .Mtls., J. Appl. Phys., 38 .(No. 3) 1967, pp. 1001-2 (QClJ82).

Bozorth, R., Ferromagnetism, New York,-1951, pp. 7-8 and 484-489 (QC753B69).

WALTER R. SATTERFIELD, Primary Examiner US. 01. X.R. 148-3157, 101, 102 

